A running condition of a vehicle changes diversely from starting the vehicle to stopping the vehicle. For example, the vehicle is accelerated after starting, a vehicle speed is kept to a constant speed when cruising, and the vehicle is also accelerated when overtaking a forerunning vehicle. In addition, the vehicle is temporarily decelerated, is temporarily stopped, climbs an upgrade, runs down a down grade, is decelerated to be stopped etc. A required driving force is varied depending on those situations, and the driving force is not required during decelerating or stopping the vehicle. According to the conventional art, however, an engine speed has to be kept to an idling speed to be maintained in a self-sustaining condition even when the driving force is not required.
The engine is used not only to generate a driving force but also to drive an alternator to generate electricity, to drive an oil pump to generate hydraulic pressure, and to drive an air conditioner compressor. Therefore, the electric power, the hydraulic pressure and air conditioning are ensured by operating the engine even during stopping or decelerating the vehicle. However, a required amount of fuel to maintain the engine speed to the self-sustaining speed is larger than a required amount of the fuel to ensure electric generation and air conditioning. In order to reduce fuel consumption according to the prior art, a fuel cut-off control is carried out to temporarily stop fuel supply to the engine during decelerating the vehicle until the rotational speed of the engine is lowered to a lowest speed possible to be reactivated by supplying the fuel thereto. For example, Japanese Patent Laid-Open No. 2002-227885 describes a clutch control device configured to allow the vehicle to coast. According to the teachings of Japanese Patent Laid-Open No. 2002-227885, in order to lighten a load to rotate the engine passively and to interrupt power transmission between the engine and drive wheels, a clutch is brought into disengagement upon satisfaction of a predetermined condition to disconnect the engine from a transmission.
Other kinds of fuel saving controls are also available in the conventional arts to reduce fuel consumption of the vehicle. To this end, specifically, necessity to activate the engine is determined based on an opening degree of an accelerator, an operating condition of a brake, a vehicle speed etc. According to those kinds of fuel saving controls, fuel supply to the engine is stopped or reduced while disconnecting the engine from a powertrain if the engine is not required to be operated. Consequently, an amount of the fuel consumed by the engine can be reduced during running.
However, during execution of the fuel cut-off control, the engine is rotated passively by an inertia force and hence the inertial energy is frictionally consumed as an engine braking force. In this situation, if the vehicle is intended to be stopped, such inertial energy can be utilized to decelerate the vehicle. By contrast, such inertial energy would be consumed wastefully if the vehicle will be driven at a constant speed or accelerated after decelerated. In turn, during execution of the coasting control, a clutch is disengaged to disconnect the engine from the powertrain so that the inertial energy will not be consumed wastefully to apply the engine braking force. During the coasting, however, the fuel is consumed to keep the engine at a self-sustaining speed. In this case, therefore, the fuel will be consumed to unnecessarily activate the engine during deceleration of the vehicle to stop. If the engine is stopped during coasting, fuel consumption can be reduced. However, if the vehicle is intended to be accelerated or propelled at a constant speed after being decelerated, the engine will have to be started and hence the fuel will be consumed. That is, if the fuel consumption could not be reduced during coasting, a fuel consumption to restart the engine may exceed the fuel reduction.
Thus, fuel consumption can be reduced during execution of the energy saving controls such as the fuel cut-off control and the coasting control. However, fuel consumption may be increased by the fuel cut-off control or the coasting control, or otherwise, other kinds of energy saving control would be more appropriate to save the fuel depending on an expected running condition within a subsequent traveling zone. According to the conventional arts, those kinds of energy saving controls are executed based on an operating condition of the vehicle, and hence the energy saving control selected during running may not necessarily be the most appropriate control to save the fuel depending on a running environment of the vehicle. Therefore, the energy saving control has to be improved to save the fuel in accordance with the running environment of the vehicle.